Liquid Fuel, from the Sun

“This is our artificial sun,” Joel Ager said, as he gestured with mock grandeur toward a metal box about the size of an old computer tower. A glowing lens, which looked like it was transplanted from a projector, shined out of a hole in its side. It was aimed at a beaker filled with water sitting a few inches away. Ager’s colleague produced a metallic toothpick-sized stick, alligator-clipped it to electrodes, and dunked it. Under the light, the submerged stick became a luminous red. “Wait for it,” Agar said. After a few seconds the red material began to emit bubbles of oxygen, slowly silvering over with tiny spheres.

The red stick was a piece of what could one day be the heart of a solar fuel generator that converts water, light, and atmospheric carbon into liquid fuel, with oxygen as its only byproduct. Built out of nano-thin layers of semiconductors that can rip water molecules into their constituent parts—hydrogen and oxygen—it rearranges their electrons into hydrogen atoms before spitting out the free oxygen that’s left over. The bubbles we’d just seen were the oxygen being released. And the light was a perfect imitation of the kind of sunlight you’d get at noon in northern latitudes.

Ager, a wiry, fast-talking scientist at the Lawrence Berkeley National Laboratory, is one of dozens of researchers here working on the device, which is inspired by the process that plants use to draw energy from sunlight. That process, photosynthesis, is unique to plant cells and a handful of other organisms. It also gives the solar fuel generator its nickname: the artificial leaf. (David Owen wrote a feature for the magazine last year about a different, related artificial-leaf project, run by the researcher Daniel Nocera.)

Billions of years ago, in Earth’s primordial oceans, a few species of bacteria evolved the ability to photosynthesize, drawing sustenance from water, carbon, and sunlight. At some point, one of these photosynthesizers was eaten by another kind of bacteria. Somehow, the photosynthesizer survived inside its would-be devourer, creating a symbiotic relationship by nourishing it with energy from the sun. When the bacteria reproduced, its daughters also contained the helpful symbiote.

As the American Museum of Natural History invertebrate-zoology curator Eunsoo Kim recently discovered, it’s likely there was even a third stage of absorption where the cell-within-a-cell was consumed by yet another bacterium. Over millennia of evolution, those bacteria became chloroplasts, the organs in plants that take care of photosynthesis. And thus the chemical process of photosynthesis spread from single-celled bacteria to plants via a multi-stage process of absorption. Every plant on Earth owes its energy-making capabilities to those bacteria, which opportunistically gobbled up what they needed from the energy-producing cells around them. Kim’s insight offers powerful evidence that solar power has spread across the Earth piecemeal, as each new species incorporated photosynthesis into its life cycle.

Ager, who told me that he looks at plants and sees their “component parts,” is especially interested in chlorophyll, a pigment found in the chloroplasts of plants and other organisms, which has the power to pass energy into other parts of the plant. It does this by absorbing blue and red light while reflecting green back into our eyes. For an artist, this process creates the lush greens of a forest; for a physicist, it’s just frustrating. “I look at what wavelengths of light it absorbs and think that I’d like to make photosynthesis more efficient,” Ager said.

Inventing a machine that behaves the way plants have for billions of years, but more efficiently, required creating a new kind of laboratory: the Joint Center for Artificial Photosynthesis (JCAP), in Berkeley, which combines basic scientific research with rapid prototyping. Chemists, physicists, and engineers—groups normally housed in separate buildings on a university campus—work together here, sharing offices and ideas. When I visited in early summer, senior researchers worked alongside undergraduates, peering into beakers and sticking rubber-gloved hands into glass-enclosed gas chambers. A machine that sprayed a single layer of atoms onto a substrate was located one lab bench away from a 3-D printer; people designing nanoscale structures can watch as their nearly invisible creations gradually become light collectors that look like shiny black tiles, which technicians will insert into a 3-D-printed solar-generator chassis the size of a chemistry textbook.

JCAP’s work is so promising that it is one of five labs in the country anointed by the Department of Energy as an “Energy Innovation Hub,” for which it received a five-year, hundred-and-twenty-two-million-dollar grant. The money will support over a hundred scientists at Berkeley and Caltech, as well as the construction of a new building at Berkeley devoted entirely to developing a solar fuel generator that makes hydrocarbon fuel.

Over the past few decades, there have been many attempts at the artificial leaf, including the ongoing project at Harvard conducted by Nocera, whose goal is to create a low-cost system that produces hydrogen fuel. But JCAP’s researchers are going beyond hydrogen; they want to use photosynthesis to make a hydrocarbon fuel like gasoline that can fill your car’s tank.

The JCAP management-team member Frances Houle, who worked her way up from a research-chemist position at I.B.M. to being a director at Lawrence Berkeley, likens what’s happening at JCAP to what I.B.M. and Bell Labs once supported in the twentieth century: basic research that sits right on the cusp of the product cycle. The artificial leaf is not mature enough to bring to market, but it’s still promising as a near-future business proposition for energy companies. While solar energy has long been a popular alternative to fossil fuels, it is currently an intermittent energy source, since it’s very difficult to store up the electricity generated by photovoltaic cells once darkness or rain falls.

The chemist Rachel Segalman, Ager’s officemate and frequent research collaborator, joined us to show off a prototype version of the solar generator that’s fresh off the 3-D printer. Plastic tubes snake into the device: one to take in water, and the other to release hydrogen that can be stored as a liquid fuel, which can be poured into barrels, shipped, and used exactly the way oil and gas are now. Segalman cautioned that what she’s designing is really just a “rough approximation” of photosynthesis. “The leaf doesn’t make fuel. It makes sugars and other things it needs to grow and sustain the plant,” she explained, adjusting the splash guard on her glasses. “We don’t need any of that. We’re going to take the leaf’s idea of turning sunlight into food, and figure out how to turn it into fuel for us.”

How would this look? At some point, if JCAP’s devices mature, we might have fields of devices coated in the reddish semiconductor material I watched bubbling under an artificial sun, slowly releasing hydrocarbon liquid fuel into barrels. Those barrels of solar fuel would gradually replace barrels of oil in a new energy regime that would be carbon-neutral and fit in with what former U.S. Secretary of Energy Steven Chu has dubbed the “glucose economy.”

Of course, Chu’s glucose economy refers mostly to biofuels drawn from actual plants. JCAP’s goal is to cut out the biological intermediary. As Segalman puts it, “Probably what makes this most like a leaf is that eventually you’ll end up with a layered sheet that you can put out in the sunlight to generate fuel.” This is nature deconstructed. But it’s also a continuation of the work that nature began, during the process of evolution that filled the oceans with mats of blue-green algae and covered our landmasses in ferns, oaks, and orchids.

Like those hungry bacteria billions of years ago, humans today are searching for a new power source. If we’re fortunate, we’ll find one where they did, by turning photosynthesis into something we can use.